The present invention relates to local area networks and, more particularly, to local area network topologies based on serial intelligent cells.
Bus Topology
Most prior art local area networks (LAN) use a bus topology as shown by example in FIG. 1. A communication medium 102 is based on two conductors (usually twisted pair or coaxial cable), to which data terminal equipment (DTE) units 104, 106, 108, 110, and 112 are connected, via respective network adapters 114, 116, 118, 120, and 122. A network adapter can be stand-alone or housed within the respective DTE.
This prior art bus topology suffers from the following drawbacks:
1. From the point of view of data communication, the medium can vary significantly from one installation to another, and hence proper adaptation to the medium cannot always be obtained.
2. The bus topology is not optimal for communication, and hence:
a) the maximum length of the medium is limited;
b the maximum number of units which may be connected to the bus is limited;
c) complex circuitry is involved in the transceiver in the network adapter;
d) the data rate is limited.
3. Terminators are usually required at the ends of the medium, thus complicating the installation.
4. Only one DTE can transmit at any given time on the bus, and all other are restricted to be listeners.
5. Complex arbitration techniques are needed to determine which DTE is able to transmit on the bus.
6. In case of short circuit in the bus, the whole bus malfunctions, and it is hard to locate the short circuit.
7. Addresses should be associated independently with any network adapter, and this is difficult to attain with bus topology.
Star Topology
A number of prior art network devices and interconnections summarized below utilize star topology.
The multiplexer is a common item of equipment used in communication, both for local area networks and wide-area networks (WAN's). It is used in order to provide access to a data communications backbone, or in order to allow sharing of bandwidth between multiple stations. As shown in FIG. 2, one side of a multiplexer 202 is usually connected to a single high data rate connection 204 (“highway”), but several such connections can also be used. The other side of multiplexer 202 has multiple low data rate connections 206, 208, 210, 212, and 214. The ellipsis . . . indicates that additional connections can be made. Each low data rate connection uses part of the bandwidth offered by the high data rate connection. These low data rate connections can be of the same type or different types, and can have different or identical data rates. The multiplexing technique most commonly used is time-domain multiplexing (TDM). However, frequency-domain multiplexing (FDM) is also used.
A popular multiplexer in use is the voice multiplexer, shown in FIG. 3. A pulse-code modulation (PCM) bus 304 handling 2.048 megabits per second, containing 30 channels of 64 kilobits per second is connected to one side of a PABX/PBX 302, and up to 30 telephone interfaces 308, 312, and 316 are connected to the other side via connections 306, 310, and 314. The ellipsis . . . indicates that additional connections can be made. In this configuration, each channel in the PCM bus can be switched or be permanently dedicated to a specific telephone line. An example of such system is disclosed in U.S. Pat. No. 3,924,077 to Blakeslee.
Similarly a small private branch exchange (PABX/PBX), as shown in FIG. 4, is widely used (usually in an office or business environment) where several outside lines 403, 404, and 405 are connected to one side of a PABX/PBX 402, and multiple telephones 408, 412, and 416 are connected to the other side via lines 406, 410, and 414, respectively. The ellipsis . . . indicates that additional connections can be made. The PABX/PBX connects an outside line to a requesting or requested telephone, and allows connection between telephones in the premises.
In the configurations described above, star topology is used in order to connect to the units to the multiplexer, which functions as the network hub. The disadvantages of star topology include the following:
1. A connection between each unit and the network hub is required, and the wiring required for this connection can involve a lengthy run.
Thus, when adding new unit, an additional, possibly lengthy, connection between the new unit and the network hub must be added.
2. No fault protection is provided: Any short circuit or open circuit will disrupt service to the affected units.
3. The multiplexer can impose extensive space and power requirements.
Computer Interfaces
Various interface standards have been established in order to allow interoperability between the PC (personal computer) or workstation and its various connected elements. These standards usually relate to both mechanical and electrical interfaces, and include industry standard architecture (ISA), extended industry standard architecture (EISA), Personal Computer Memory Card Industry Association (PCMCIA), intelligent drive electronics (IDE), small computer system interface (SCSI), and others. Each added hardware unit usually utilizes a specific software driver for interoperability with the specific platform. These protocols are applicable to small distances only, and allow units to be housed within or nearby the PC or workstation enclosures. For example, equipping a PC for video capture could involve a plug-in ISA card housed within the PC on the motherboard, a video camera connected to the card, and a software driver. This configuration does not allow remote video monitoring.